Quantum knots observed for the first time

By Tereza Pultarova

Published Monday, January 18, 2016

Quantum knots have been observed for the first time by a team of physicists.

These knots, unlike knots on ropes or strings, have both of their ends tied together. Described in the latest issue of the Nature Physics journal, the quantum knots have been created by a team from the Aalto University in Finland and Amherst College, USA.

“To make this discovery we exposed a Rubidium condensate to rapid changes of a specifically tailored magnetic field, tying the knot in less than a thousandth of a second,” said Amherst College Professor David Hall.

“After we learned how to tie the first quantum knot, we have become rather good at it. Thus far, we have tied several hundred such knots.”

The scientists tied the knot by squeezing the structure into the condensate from its outskirts. This required them to initialize the quantum field to point in a particular direction, after which they suddenly changed the applied magnetic field to bring an isolated null point, at which the magnetic field vanishes, into the centre of the cloud. Then they waited for less than a millisecond for the magnetic field to do its trick and tie the knot.

“For decades, physicists have been theoretically predicting that it should be possible to have knots in quantum fields, but nobody else has been able to make one,” said Mikko Möttönen from Aalto University, who led the research.

“Now that we have seen these exotic beasts, we are really excited to study their peculiar properties. Importantly, our discovery connects to a diverse set of research fields including cosmology, fusion power and quantum computers.”

Quantum knots only exist in a field that assumes a certain direction at every point of space. The field segregates into an infinite number of linked rings, each with its own field direction. The resulting structure is topologically stable as it cannot be separated without breaking the rings. In other words, one cannot untie the knot within the superfluid unless one destroys the state of the quantum matter.

“This is the beginning of the story of quantum knots,” Möttönen added. “It would be great to see even more sophisticated quantum knots to appear such as those with knotted cores. Also it would be important to create these knots in conditions where the state of the quantum matter would be inherently stable. Such systems would allow for detailed studies of the stability of the knot itself.”